update irradiance.reindl docs

[RDaxini]

• Partially closes Missing Descriptions for Formula Components in haydavies #2183, related to Klucher diffuse sky docstring inconsistency #1405
• I am familiar with the contributing guidelines
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

Docs here

[RDaxini]

I have personally never used "BHI" before but I did swap out the I_hb and I_h since BHI/GHI etc are what are in the pvlib user guide. The original equation also already used "DHI", so it's consistent with the variable naming style. Overally, I think I probably lean towards using shorter symbols instead, like I_xx, but I see how DHI/GHI etc is easier to understand so I think it would help to hear other views on this.
This also relates to PRs #2191 and #2192

[RDaxini]

Marked as draft until #2191 has been addressed to avoid duplicate reviews due to similarities between the changes made in these PRs

[AdamRJensen]

Thanks @RDaxini!